Method for adjusting a brake system of a vehicle

ABSTRACT

A method for controlling a brake system for automotive vehicles including a vehicle speed control unit permitting the adjustment of a predetermined vehicle speed by way of an automatic intervention into brake control, wherein upon a braking request of the vehicle speed control unit and, simultaneously, a braking request of the driver the function of the vehicle speed control unit is deactivated, wherein the pressure (P x ) introduced into the wheel brake due to the braking request of the vehicle speed control unit is reduced according to a predetermined function upon the braking request of the driver, wherein preferably a resulting brake pressure (P res ) is adjusted in the wheel brake of the vehicle which is determined by the formula 
     
       
         P res =P F +P X[1] −(P F *P X[1] )/constant Z.

TECHNICAL FIELD

The present invention generally relates to vehicle brake systems, andmore particularly relates to a method for controlling a brake system forautomotive vehicles including a vehicle speed control unit permittingthe adjustment of a predetermined vehicle speed by way of an automaticintervention into brake control.

BACKGROUND OF THE INVENTION

Brake control systems including a vehicle speed control unit whichadjusts a predetermined vehicle speed by way of an automatic brakeintervention are known in the art. Depression of the brake pedal by thedriver causes the vehicle speed control unit to change from theactivated into the deactivated mode, with the result that an automaticbrake intervention operation performed by the vehicle speed control unitis terminated. The pressure which recently prevailed in the wheel brakesdue to the automatic brake intervention is reduced until the brakepressure introduced into the wheel brakes rises again due to applicationof the brake pedal. This causes a jerk (discontinuity) in the vehicledeceleration.

EP 0 867 349 discloses a method intended to prevent this break in thevehicle deceleration. It is proposed therein that after the deactivationof the vehicle speed control unit at the beginning of the application ofthe brake pedal, the actual pressure which was adjusted the last by thevehicle speed control unit is maintained or continuously adapted to ahigher nominal pressure demanded by the driver, or a total pressure isadjusted which corresponds to the sum of the actual pressure and thenominal pressure. The brake pressure predetermined by the vehicle speedcontrol unit is not reduced when the brake pedal is applied, but rathermaintained or increased, with the aim of providing jerkfree transitionsfrom an activated mode into a deactivated mode of the vehicle speedcontrol unit.

In a brake system with an active, electrically actuated brake forcebooster, the brake pedal is moved along with an automatic brakeintervention of the vehicle speed control unit. When the driver wishesto brake more vigorously or deactivate the vehicle speed control unit,he/she will apply the brake pedal at that point and with the force thatcorresponds to the brake pressure in the wheel brake. In a so-called‘brake-by-wire brake system’ where the transmission of the driver'sbraking request from the brake pedal to the hydraulic actuators, when anelectrohydraulic brake (EHB) is concerned, or to the electromechanicactuators, when an electromechanic brake (EMB) is concerned, is effectedelectrically, the brake pedal remains in the inactive position. Thetake-over by the driver during a braking operation initiated by thevehicle speed control unit is not transparent for the driver, especiallyin a brake-by-wire brake system.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to improve the method according tothe state of the art and, especially for a brake-by-wire brake system,to render the take-over of the control of the braking operation from anautomatic brake intervention of the vehicle speed control unit to abrake actuation determined by the driver comfortable and alsotransparent.

SUMMARY OF THE INVENTION

The essence of the present invention resides in that after the functionof the vehicle speed control unit is deactivated in the event of abraking request of the vehicle speed control unit and, simultaneously, abraking request of the driver, the pressure (P_(A)) introduced into thewheel brake due to the braking request of the vehicle speed control unitis reduced according to a predetermined function (P_(X)) upon thebraking request of the driver.

The reduction of the pressure (P_(A)) introduced into the wheel brake inresponse to the braking request of the vehicle speed control unitpermits the driver to recognize when the application of the brake pedalleads to a brake intervention controlled by the driver. The reduction isperformed according to a predetermined function which is so chosen thatthe brake application is comfortable and also transparent for the driverand, in addition, a maximum high deceleration is ensured in an emergencysituation (panic stop).

According to the present invention, an at least approximately linearramp function is predetermined as the function.

The predetermined function has a mean (negative) gradient (m_(PX)) of(20 bar/5 sec) to (30 bar/0.5 sec), preferably of (20 bar/2 sec) to (30bar/1 sec) as disclosed by the present invention.

In the present invention, the function is changed in accordance with thebraking request of the driver.

According to the present invention, a function is predetermined upon arequest of the driver for quick braking of the vehicle causing quickreduction of the pressure (P_(A)) introduced into the wheel brake due tothe braking request of the vehicle speed control unit, and upon arequest of the driver for slow braking of the vehicle a function ispredetermined causing slow reduction of the pressure (P_(A)) introducedinto the wheel brake due to the braking request of the vehicle speedcontrol unit.

It is arranged for by the present invention that a mean (positive)gradient (m_(PF)) of the brake pressure is determined from the brakingrequest of the driver, and therefrom the mean (negative) gradient(m_(PX)) of the predetermined function is defined by the formulam_(PX)=−m_(PF)*constant K, until a top limit value of 100 bar/sec). Thismeans a maximum mean (negative) gradient (m_(PX)) and, thus, a maximumpressure reduction speed of (100 bar/sec) is predetermined. The constantK has preferably a value in the range from 1 to 2, particularlypreferred is a value of 1.5.

Because it is desired that brake pressure reduction is carried out ineach case relatively quickly after deactivation of the vehicle speedcontrol unit, it is arranged for that the value for the mean (negative)gradient (m_(PX)) of the predetermined function does not fall below aminimum limit value (lowest pressure reduction speed) in a range of (20bar/5 sec) until (20 bar/2 sec), preferably 20 bar/3 sec).

It is provided according to the present invention that a resulting brakepressure (P_(res)) in the wheel brake of the vehicle is adjusted inaccordance with the pressure (P_(F)) introduced into the wheel brake dueto the braking request of the driver, and the pressure (P_(X))introduced into the wheel brake due to the predetermined function.

According to an embodiment of the present invention, a resulting brakepressure (P_(res)) is adjusted in the wheel brake of the vehicle whichcorresponds to the sum of a pressure (P_(F)) introduced into the wheelbrake due to the braking request of the driver and the pressure (P_(X))introduced by the predetermined function into the wheel brake.

According to another embodiment of the present invention, a resultingbrake pressure (P_(res)) is adjusted in the wheel brake of the vehiclewhich corresponds to the higher of the two pressures, i.e., the pressure(P_(F)) introduced into the wheel brake due to the braking request ofthe driver or the pressure (P_(X)) introduced by the predeterminedfunction into the wheel brake.

In another embodiment of the present invention, a resulting brakepressure (P_(res)) is adjusted in the wheel brake of the vehicle whichcorresponds to a value that is lower than the sum of the pressure(P_(F)) introduced into the wheel brake due to the braking request ofthe driver and the pressure (P_(X)) introduced by the predeterminedfunction into the wheel brake, and which is higher than the higher oneof the two pressures, i.e., the pressure (P_(F)) introduced into thewheel brake due to the braking request of the driver or the pressure(P_(X)) introduced by the predetermined function into the wheel brake.

The latter embodiment of the present invention is especially preferredbecause it renders the brake pressure take-over by the driver especiallycomfortable and also transparent. According to the present invention, ithas proved particularly favorable to adjust a resulting brake pressure(P_(res)) in the wheel brake of the vehicle which corresponds to a valuethat is determined by the formulaP_(res)=P_(F)+P_(X)−(P_(F)*P_(X))/constant Z.

According to the present invention, the constant Z (adjustment factor)has a value in the range of 20 bar to 100 bar, preferably 30 bar to 70bar.

It is in the sense of the present invention that the brake pressure inthe wheel brake is generated by independent energy, in particular, by ahydraulic accumulator and a motor-and-pump assembly. Preferably, thebrake pressure request of the driver is sensed by means of a pedaltravel simulator and associated sensors and transmitted electronicallyto a brake control unit (brake-by-wire brake system).

Further, the object is achieved by a device for controlling a brakesystem for automotive vehicles with a vehicle speed control unit whichincludes means to adjust a predetermined vehicle speed by means of anautomatic intervention into the brake control and means to deactivatethe function of the vehicle speed control unit in the event of a brakingrequest of the vehicle speed control unit and, simultaneously, a brakingrequest of the driver, the said device being characterized by theprovision of means to reduce the pressure (P_(X)) introduced into thewheel brake due to the braking request of the vehicle speed controlaccording to a predetermined function unit in the event of the brakingrequest of the driver, and by the provision of means to adjust inparticular a resulting brake pressure (P_(res)) in the wheel brake ofthe vehicle that is determined by the formulaP_(res)=P_(F)+P_(X)−(P_(F)*P_(X))/constant Z.

Preferably, an independent-force-actuated, preferably electronicallycontrollable vehicle brake, especially brake-by-wire vehicle brake,includes the device of the present invention for controlling the vehiclebrake system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of the time dependence of the pressure (P_(A)) that isintroduced into the wheel brake by the vehicle speed control unit, ofthe pressure (P_(X)) introduced by the predetermined function into thewheel brake, of the pressure (P_(F)) introduced into the wheel brake dueto the braking request of the driver, and of the resulting brakepressure (P_(res)) calculated according to the formulaP_(res)=P_(F)+P_(X)−(P_(F)*P_(X))/constant Z for a predeterminedpressure P_(X) of 20 bar and a final pressure P_(F) of likewise 20 bar.

FIG. 2, similar to FIG. 1, shows the time dependence of P_(A), P_(X),P_(F), and P_(res), wherein P_(res) was calculated according to the sameformula, the output pressure P_(A) also amounts to 20 bar, but the finalpressure P_(F) amounts to 30 bar.

FIG. 3 shows the time dependence of P_(A), P_(X), P_(F), and P_(res)similar to FIGS. 1 and 2, wherein P_(res) was calculated according tothe same formula and the output pressure P_(A) also amounts to 20 bar,but a lower final pressure P_(F) of 5 bar prevails.

FIG. 4 schematically shows possible components of the device of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment of FIG. 1 illustrates the pressure variation of thepressure (P_(A)) (line with short dashes) introduced into the wheelbrake by the vehicle speed control unit, of the pressure (P_(X))introduced into the wheel brake by the predetermined function (line withlong dashes), of the pressure (P_(F)) introduced into the wheel brakedue to the braking request of the driver (dot-dash line), and of theresulting brake pressure (P_(res)) (solid lines) plotted against thetime (t). The pressure P_(res) adjusted in the wheel brake of thevehicle was determined according to the formulaP_(res)=P_(F)+P_(X)−(P_(F)*P_(X))/constant Z. As a constant Z, a valueof Z=60 (see above, solid line P_(res,Z=60)) and of Z=40 (see bottomsolid line P_(res,z=40)) is predetermined.

Until time t₀, the brake pressure P corresponds to the pressure (P_(A))(line with short dashes) introduced into the wheel brake by the vehiclespeed control unit, because there is no braking request of the driver.

A braking request of the driver is recognized at time t₀. The vehiclespeed control unit is deactivated, and the current braking request ofthe vehicle speed control unit (P_(A)) is stored or temporarily stored.The braking request may be detected e.g. in a brake system controlled byindependent energy by a contact with the brake pedal or when the brakepedal is depressed over a predetermined distance or with a predeterminedforce.

Starting from time t₀, based on the stored value of the braking requestof the vehicle speed control unit, this brake pressure (P_(A)) isreduced until the value zero (line P_(X)) according to an at leastapproximately linear ramp function. The current value of P_(X) andcurrent value of the braking request by the driver (P_(F)) is used as acontrol quantity for adjusting the resulting brake pressure. Theresulting brake pressure (P_(res)) is adjusted according to the formulaP_(res)=P_(F)+P_(X)−(P_(F)*P_(X))/constant Z. The brake pressure P_(res)calculated according to this formula is illustrated in FIG. 1 for avalue of Z=60 (top solid line P_(res,Z=60)) and Z=40 (bottom solid lineP_(res,Z=40)).

As can be seen in FIG. 1, due to the inventive function for controllingthe resulting brake pressure (P_(res)) and for the given limitingconditions, after time to the brake pressure P_(res) is initiallyreduced relatively slowly until a defined value, is subsequentlyreincreased relatively slowly at time t₁ when P_(F)+P_(X) roughly adoptthe same value, and finally is brought into conformity with the valueP_(F) at time t₂. Due to the control of the present invention, thedriver feels the deactivation of the vehicle speed control unit due tothe decrease of the brake pressure commencing t₀, on the one hand. Onthe other hand, the driver feels the subsequent rise of the brakepressure commencing t₁ which indicates the take-over of the brakeintervention corresponding to the driver's brake pedal depression.Commencing time t₂, the brake pressure is determined exclusively by thedriver because P_(X) adopted the value zero and, according to the aboveequation, the control is effected only according to the driver'srequest. Here, the pedal feeling to which the driver is accustomed isreached again, where a defined pedal position or a defined pedalresetting force causes a defined brake pressure or a defined vehicledeceleration—under the given limiting conditions, such as e.g. wheelslip conditions.

In the variation of P_(A), P_(X), P_(F), and P_(res) illustrated in FIG.2, a situation is assumed where the driver wishes a quick decelerationof the vehicle. In accordance with the relatively steep rise of thebrake pressure (P_(F)) predefined by the driver, the linear rampfunction also declines with a relatively great (negative) gradient, thatmeans relatively quickly. The result is that the resulting brakepressure P_(res) approaches the driver's specification P_(F) relativelyquickly, and the brake pressure take-over can be felt by the driver attime t₁ at the latest. This is illustrated in FIG. 2 for a value of Z=70(top solid line P_(res,Z=70)) and of Z=30 (bottom solid lineP_(res,Z=30)).

Following is an adaption of the ramp function for P_(X) to the driver'sspecification which is characterized especially by the speed at whichthe driver depresses the brake pedal, to the end that the ramp for P_(X)will decline more quickly in the event of a quicker depression, with theresult that the driver more quickly determines directly the resultingbrake pressure P_(res). This is also shown in a comparison of thevariations of the pressure curves P_(X), P_(F), and P_(res) in FIG. 1(slower pedal depression by the driver) and in FIG. 2 (quicker pedaldepression by the driver).

FIG. 3 shows the variation of P_(A), P_(X), P_(F), and P_(res) for asituation which is contrary to the situation shown in FIG. 2. This isbecause here the driver's speed of pedal depression is relatively low,and the brake pressure maximally desired by the driver is relativelylow. Corresponding to the driver's request, commencing t₀, the speedcontrol unit is initially deactivated. Then P_(X) decreases according toa predetermined ramp function. The resulting brake pressure P_(res)which is determined according to the mentioned formula and shall beadjusted will decline relatively quickly in this case so that finally,at time t₂, where P_(X) adopts the value zero, the resulting brakepressure P_(res) is exclusively defined by the brake pressure P_(F)determined by the driver. This is illustrated in FIG. 3 for a value ofZ=70 (top solid line P_(res,Z=70)) and of Z=30 (bottom solid lineP_(res,Z=30)). In this case, too, initially the deactivation of thespeed control unit at time t₀ can be felt by the driver due to thereduction of the pressure on the wheel brakes (lines P_(res)). Also, thetransition to the brake pressure delivery which is exclusivelydetermined by the driver becomes transparent to the driver commencingtime t₂ because the pedal feeling he/she is used to will begin at thispoint of time.

It becomes apparent from the previously mentioned examples that, on theone hand, the deactivation of the speed control unit and the brakepressure buildup corresponding to brake pedal depression is transparentto the driver by means of the present invention and that, on the otherhand, a comfortable adaption of the brake pressure (P_(A)) which isinitially predetermined by the speed control unit to the brake pressurelevel (P_(F)) desired by the driver is achieved.

Possible components of the device of the present invention for abrake-by-wire brake system are represented in FIG. 4. A brake pedal 1with at least one travel sensor 2 for sensing the pedal position isconnected to a master brake cylinder 3. Master brake cylinder 3 isconnected to wheel brake 6 of the vehicle by way of a hydraulic line 5that can be closed by means of a valve 4. The normally open valve 4 isclosed in a normal service brake function, and the required brakepressure is generated and controlled by means of an independent energysource 8 with a hydraulic control circuit 7, in particular, amotor-pump-accumulator unit 8 and electromagnetic valves 7. Themotor-pump-accumulator unit 8 and the electromagnetic valves 7 arecontrolled by corresponding signals of a control unit 10 by way of line9. A signal for the braking pressure request of the driver, sensed bymeans of the at least one sensor 2, is sent to the control unit 10 byway of line 11. By way of line 14, control unit 10 is connected to aspeed control unit 15 by which, in the activated mode, a predeterminedvehicle speed is adjustable by means of an automatic brake interventionby way of actuation of the controllable brake system components 7, 8.Further, a pedal travel simulator 16 is assigned to the brake pedal 1 toproduce a pedal feeling which is ‘familiar’ to the driver. Further, thesignal for the wheel rotational behavior sensed by a wheel speed sensor12 is sent to the control unit 10 by way of line 13. E.g. the currentown speed of the vehicle and said's acceleration or deceleration can besensed by means of wheel speed sensor 12. According to the signals sentto the control unit 10, corresponding to the method of the presentinvention, the current deceleration of the vehicle is sensed, evaluatedas a control quantity, and the vehicle is slowed down corresponding to apredetermined function in accordance with the control quantity. In doingso, preferably a resulting brake pressure (P_(res)) is adjusted in thewheel brake 6 of the vehicle which is determined by the formulaP_(res)=P_(F)+P_(X)−(P_(F)*P_(X))/constant Z. Only in a case ofemergency will brake pressure be applied to the wheel brake 6 by way ofthe master brake cylinder 3, the normally open valve 4, and line 5. Thisso-called hydraulic punch-through ensures an auxiliary braking functionin this electronically controlled brake-by-wire brake system. Inaddition, reference is made to DE 197 18 533 A1 for a possible detailedarrangement of the hydraulic components of the brake-by-wire brakesystem and for said mode of operation.

What is claimed is:
 1. Device for controlling a brake system forvehicles, comprising: a vehicle speed control unit which includes meansto adjust a predetermined vehicle speed by means of an automaticintervention into a brake controller and means for deactivating thefunction of the vehicle speed control unit in the event of asimultaneous braking request from the vehicle speed control unit and abraking request of the driver, wherein the device includes means toreduce the pressure P_(A) introduced into the wheel brake due to thebraking request of the vehicle speed control unit according to apredetermined function P_(X) in the event of the braking request of thedriver, and means for adjusting a resulting brake pressure P_(res) inthe wheel brake of the vehicle that is determined by the formulaP_(res)=P_(F)+P_(X)−(P_(F)*P_(X))/constant Z, where: P_(F) is thepressure introduced into the wheel brake due to the brake request of thedrive; P_(X) is the pressure introduced into the wheel brake by thevehicle speed control according to a predetermined function.
 2. Methodfor controlling a vehicle brake system, comprising the steps of:permitting a vehicle speed control unit to adjust a predeterminedvehicle speed by way of an automatic intervention into a brake control,deactivating the vehicle speed control unit, upon a braking request of avehicle driver, the deactivation of the vehicle speed control unit isaccomplished by reducing a pressure P_(A) of the brake fluid pressureintroduced into a wheel brake due to a braking request of the vehiclespeed control unit according to a predetermined function P_(X) upon thebraking request of the driver, wherein an approximately linear rampfunction is used as the predetermined function.
 3. Method as claimed inclaim 2, wherein the predetermined function has a mean negative gradientm_(PX) of 20 bar/5 sec to 30 bar/0.5 sec.
 4. Method as claimed in claim2, wherein the function is variable, as a function of the brakingrequest of the driver.
 5. Method as claimed in claim 2, the methodfurther includes the step of: changing the predetermined function inaccordance with the braking request of the driver, wherein upon arequest of the driver for quick braking of the vehicle, thepredetermined function is used causing quick reduction of the pressureP_(A) introduced into the wheel brake due to the braking request of thevehicle speed control unit, and upon a request of the driver for slowbraking of the vehicle, the predetermined function is used causing slowreduction of the pressure P_(A) introduced into the wheel brake due tothe braking request of the vehicle speed control unit.
 6. Method aschimed in claim 2, the method further includes the steps of: changingthe predetermined function in accordance with the braking request of thedriver, wherein upon a request of the driver for quick braking of thevehicle, the predetermined function is used causing quick reduction ofthe pressure P_(A) introduced into the wheel brake due to the brakingrequest of the vehicle speed control unit, and upon a request of thedriver for slow braking of the vehicle, the predetermined function isused causing slow reduction of the pressure P_(A) introduced into thewheel brake due to the braking request of the vehicle speed controlunit, wherein a mean positive gradient m_(PF) of the brake pressure isdetermined from the braking request of the driver, and therefrom themean negative gradient m_(PX) of the predetermined function is definedby the formula m_(PX)=−m_(PF)*constant K, until a top limit rate of 100bar/sec is attained.
 7. Method as claimed in claim 2, wherein aresulting brake pressure P_(res) in the wheel brake of the vehicle isadjusted in accordance with the pressure P_(F) introduced into the wheelbrake due to the braking request of the driver and the pressure P_(X)introduced into the wheel brake due to the predetermined function. 8.Method as claimed in claim 2, further including the step of: adjusting abrake pressure P_(res) in the wheel brake of the vehicle as a functionof the sum of the pressure P_(F) introduced into the wheel brake due tothe braking request of the driver and the pressure P_(X) introduced intothe wheel brake due to the predetermined function.
 9. Method as claimedin claim 2, further including the step of: adjusting a resulting brakepressure P_(res) in the wheel brake of the vehicle as a function of thegreater of the pressure P_(F) introduced into the wheel brake due to thebraking request of the driver and the pressure P_(X) introduced into thewheel brake due to the predetermined function.
 10. Method as claimed inclaim 2, further including the step of: adjusting a resulting brakepressure P_(res) in the wheel brake of the vehicle as a function of thepressure P_(F) introduced into the wheel brake due to the brakingrequest of the driver and the pressure P_(X) introduced into the wheelbrake due to the predetermined function, wherein a resulting brakepressure P_(res) is adjusted in the wheel brake of the vehicle whichcorresponds to a value that is lower than a sum of the pressure P_(F)introduced into the wheel brake due to the braking request of the driverand the pressure P_(X) introduced by the predetermined function into thewheel brake, and which is higher than the higher one of the twopressures.
 11. Method as claimed in claim 2, further including the stepof: adjusting a resulting brake pressure P_(res) in the wheel brake ofthe vehicle as a function of the pressure P_(F) introduced into thewheel brake due to the braking request of the driver and the pressureP_(X) introduced into the wheel brake due to the predetermined function,and wherein the method further includes the step of: adjusting aresulting brake pressure P_(res) in the wheel brake of the vehicle as afunction of a value that is lower than a sum of the pressure P_(F)introduced into the wheel brake due to the braking request of the driverand the pressure P_(X) introduced by the predetermined function into thewheel brake, and which is higher than the highest one of the twopressures, wherein a resulting brake pressure P_(res) is adjusted in thewheel brake of the vehicle which corresponds to a value that isdetermined by the formula P_(res)=P_(F)+P_(X)−(P_(F)*P_(X))/constant Z.12. Method as claimed in claim 2, further including the step of:adjusting a resulting brake pressure P_(res) in the wheel brake of thevehicle as a function of the pressure P_(F) introduced into the wheelbrake due to the braking request of the driver and the pressure P_(X)introduced into the wheel brake due to the predetermined function, andwherein the method further includes the step of: adjusting a resultingbrake pressure P_(res) in the wheel brake of the vehicle whichcorresponds to a value that is lower than a sum of the pressure P_(F)introduced into the wheel brake due to the braking request of the driverand the pressure P_(X) introduced by the predetermined function into thewheel brake, and which is higher than the highest one of the twopressures, and the method further comprising a resulting brake pressureP_(res) is adjusted in the wheel brake of the vehicle which correspondsto a value that is determined by the formulaP_(res)=P_(F)+P_(X)−(P_(F)*P_(X))/constant Z, wherein the constant Z hasa value in the range of 20 bar to 100 bar.
 13. Method as claimed inclaim 2, wherein the brake pressure in the wheel brake is generated byindependent energy.
 14. Method as claimed in claim 2, further includingthe step of: generating the brake pressure in the wheel brake byindependent energy, wherein the brake pressure request of the driver issensed by means of a pedal travel simulator and associated sensors andtransmitted electronically to a brake control unit.